Tower wont connect to any monitor

the tower worked fine a few months ago, but it hasnt been used in a few months, not touched nothing at all just sitting there...

anyways i go to try and use it i plug a monitor up to it, the computer turns on lights up and runs but IT DOES NOT CONNECT TO THE MONITOR!

The monitors says "self check, your monitor is working correctly, please check cables and connection" well the Monitor works fine ive used it with 2 other computers so i knew it wasnt the monitor i tryed using the tower with another monitor i had SAME PROBLEM monitor works but doesnt connect and computer turns on, no connection to monitor.

i thought it might be the Grahpics card so i changed it (the card is not conncted to the MOtherBoard there is a slot one of them kinds)
Still did not work with diff grahpic card..

It sounds like it's not booting up at all.
Which could be any number of things.

Remove any components and re-seat components and cables that are not needed to boot to POST screen. (including hard drives and cd drives, extra memory)
If it still doesn't boot you need to troubleshoot the system.
I would start with the power supply, then memory, video card, mother board.

It sounds like it's not booting up at all.
Which could be any number of things.

Remove any components and re-seat components and cables that are not needed to boot to POST screen. (including hard drives and cd drives, extra memory)
If it still doesn't boot you need to troubleshoot the system.
I would start with the power supply, then memory, video card, mother board.

and if u read i already replaced the graphic card and it still didnt work